1. Field of the Invention
The present invention relates to an automatic focus detecting camera. More particularly, the present invention relates to an automatic focus detecting camera of a single-lens reflex type capable of performing preview operation to allow a photographer to see an image in a finder by stopping down an aperture diaphragm to a predetermined aperture value so that he can directly know an actual depth of field when the aperture diaphragm is stopped down to the predetermined aperture value.
2. Description of the Prior Art
There are various automatic focus detecting systems. As an example, there is known an automatic focus detecting apparatus of a system for detecting a focusing position of an objective lens by the overlap of images of the same portion of an object caused by light rays passing through two portions of an exit pupil of the objective lens both of which are symmetrical with regard to the optical axis of the objective lens. In such an automatic focus detecting apparatus, a portion contributing to focus detection in the objective lens is caused to be covered by the aperture diaphragm if the diaphragm is stopped down to a certain extent.
As a result, automatic focus detection can not be made if the aperture diaphragm is stopped down exceedingly.
Therefore, in an "Automatic Focusing Position Detecting Apparatus" disclosed in U.S. Pat. No. 4,387,975, focus detection is interrupted at the time of preview operation because shading is caused in the incident light applied to the light receiving elements when the diaphragm is stopped down by the preview operation in a focus detection optical system of an exit pupil divisional type. In addition, in a single-lens reflex camera disclosed in Japanese Patent Laying-Open Gazette No. 124330/1982, focus detection is stopped at the time of preview operation and the state of focus detection before the preview operation is maintained during the preview operation. Those prior art techniques intend to avoid occurrence of erroneous focus detection which would be caused by automatic focus detection effected in spite of the state where focus detection can not be made, and which would result in a photograph out of focus if the photograph were taken under that condition. Those prior art systems make it possible to prevent a photograph from being taken out of focus because a image to be photographed is previewed if a desired aperture value corresponds to a diameter smaller than a certain limit value. However, focusing operation needs to be made manually at the time of preview operation in those prior art systems and accordingly the essential feature as an automatic focus detecting camera is lost by such preview operation.